Whether taking a basic approach or performing extensive data gathering and analysis, there's a core set of fundamental segmentations that drive relevance and consistently outperform across the most successful e-mail marketing programs. In this article on the Multichannel Merchant website, Millie Park sums them up:

E-mail behavior: by leveraging the behavioral data collected from your e-mail programs, you can design campaigns with greater relevance to your targets and generate a higher number of clicks.

Purchasing behavior: of course, customers with the highest RFM numbers should be marketed to in a distinct way. But segmenting by one element alone can increase hit rates. For example, if you have a customer who makes a purchase every three months, you should market to her near the end of this cycle, offering discounts to entice her to not only buy again, but possibly increase spending.

Purchase category: you already know what customers have purchased in the past, so from this you can deduce what may interest them in the future.

Preference center and survey data: preference center data and/or data obtained from surveys is infrequently used or leveraged to its fullest, yet it can serve as strong basis for customizing programs and improving e-mail relevancy.

Demographic or psychographic data: you may not already be tracking information as detailed as a customer's gender. But you can easily glean it from other data you've collected, directly ask customers for it or purchase it. For example, results from a brief survey asking a customer for his zip code can enable you to segment him by geographic location and gain insight into what's most relevant to him.

You don't need complex customer models and sophisticated publishing grids to effectively segment customers. Small incremental changes such as tailoring subject lines can have a big impact on the relevance of your e-mail marketing and your bottom line.